It is prevalent in the prior art to prepare isopropanol from propylene, but it has never been a practice to prepare propylene from isopropanol. Nevertheless, the current situation where acetone is by-produced in surplus in the phenol preparation by the cumene process imposes it under consideration to prepare propylene from isopropanol resulting from hydrogenation of acetone. More particularly, the mainstream of the current industrial phenol preparation is the cumene process. The phenol preparation by the cumene process involves forming cumene from benzene and propylene, automatically oxidizing the cumene into cumene hydroperoxide, and subjecting the cumene hydroperoxide to decomposition in the presence of an acid catalyst to form phenol and acetone. The phenol preparation by the cumene process is a cost efficient method insofar as the demands for phenol and acetone keep balance.
The demand for acetone is decreasing in the recent years. Although the use of acetone as a starting material toward methyl methacrylate was a major demand for acetone, the methyl methacrylate preparation was switched to the use of compounds having 4 carbon atoms. As a consequence, there is a decreasing demand for acetone.
It is thus desired to make efficient use of acetone without leaving acetone as a by-product. One promising attempt is to convert acetone into isopropanol and dehydrating isopropanol back to propylene. It is now required to develop a process for preparing propylene from isopropanol which has never been sought in the previous circumstances.
Among classical methods for the preparation of olefins, it is well known to subject alcohols to dehydration in the presence of strong acids such as sulfuric acid, phosphoric acid, perchloric acid, phosphotungstic acid, and phosphomolybdic acid. Although the preparation of olefins is predominantly based on naphtha cracking in recent years, a variety of proposals have been made on alcohol dehydration to prepare olefins for the purposes of rendering diverse reactants available for olefin preparation and producing olefins of high purity. For example, it was proposed to produce ethylene by dehydrating ethanol as disclosed in Japanese Patent Publication Nos. 40057/1984 and 19927/1984 and to produce high purity isobutylene by dehydrating tertiary butanol as disclosed in Japanese Patent Publication No. 23771/1986 and Japanese Patent Application Kokai No. 26/1986. As to the catalyst for assisting in preparation of ethylene through dehydration of ethanol, it is proposed to use solid acid catalysts such as alumina, silica, silicaalumina, zeolites and solid phosphoric acids as disclosed in Japanese Patent Application Kokai No. 34929/1989.
As for the conversion of isopropanol to propylene through dehydration, however, no appropriate process is available other than the above-mentioned strong acid catalyzed dehydration. Although it might occur to those skilled in the art to apply the conventional technique for dehydrating ethanol or tertiary butanol, no presumption can be made because the end products, ethylene or isobutylene and propylene are considerably different in nature.
In general, the use of strong acid catalysts has problems that the reactor used must be of expensive corrosion-resistant material and the outgoing used acid must be disposed of with difficulty. In addition, the once produced olefins can react in the presence of strong acids, for example, they can polymerize into higher molecular weight polymers or isomerize into undesired compounds, resulting in reduced yields of the end olefins. Further, propylene is more active than ethylene and isobutylene and susceptible to polymerization. This suggests that it is impossible to apply the methods described in Japanese Patent Publication Nos. 40057/1984, 19927/1984, 23771/1986, and 26/1986 to the preparation of propylene.
On the contrary, the method for the preparation of ethylene through dehydration of ethanol in the presence of solid acid catalysts described in Japanese Patent Application Kokai No. 34929/1989 is advantageous in that a simple reactor of less expensive material may be used because the catalysts are not corrosive. However, silica-alumina, zeolites, and solid phosphoric acids are strongly acidic. If isopropanol in gas state is passed through a reactor charged with such a strongly acidic catalyst at relatively low temperatures of 250.degree. to 300.degree. C., as much as about 30% of the resulting propylene is further catalyzed into polymers. Formation of substantial amounts of high molecular weight polymeric by-products results in reduced yields of propylene. Dehydration of isopropanol into propylene is a substantial endothermic reaction requiring a reaction temperature of at least 250.degree. C., at which the above-mentioned strong acid catalysts are unfeasible as industrial catalysts.
The alumina catalysts are known to be effective for dehydration of ethanol. However, if commercially available alumina catalysts are applied to dehydration of isopropanol, propylene would not be obtained in high yields. It would be possible to increase the percent conversion of isopropanol by raising the reaction temperature to 450.degree. C. or higher. However, propylene is obtained in low yields since cracking reaction occurs simultaneous with dehydration at elevated temperatures so that the resulting propylene contains substantial amounts of impurities.